Polycarboxylic acid-type highly water-absorbing resins, especially polyacrylic acid-type polymers, which have been used in a large quantity for diapers, menstrual goods, etc., are also brought into use in the field of agriculture due to their inexpensiveness and excellent water-retaining ability.
For example, hydrogels of the polyacrylic acid-type polymers have been used as a support for fluid seeding; or a water-retaining support for virescence engineering, water-saving cultivation, or cultivation on sandy soil, by utilizing their water-retaining ability.
However, it has been recognized that the conventional polyacrylic acid-type hydrogels affect the growth of a plant, and particularly, they cause a marked inhibition of the root origination and root elongation when the hydrogels are used in an amount exceeding their appropriate amount (Kazuo Kawashima, et al., "Influences of Highly Water-Absorbing Polymer Materials on Initial Growth of Crops," Sand Dune Research, 31(1), 1-8, 1984).
Particularly, when the conventional polyacrylic acid-type hydrogel is used as a support for tissue culture, a support for fluid seeding, and a support for virescence engineering, a plantlet, seed, etc., of a plant are caused to directly contact the high-concentration polyacrylic acid-type hydrogel, and therefore its root origination and root elongation are markedly inhibited, whereby the use of the polyacrylic acid-type hydrogel is severely restricted. It has also been recognized that, in a case where the conventional polyacrylic acid-type hydrogel is used as a water-retaining support for farm or field cultivation, the elongation of the root is inhibited when the concentration of the polymer in the vicinity of the root is increased so as to enhance the effect of the water-retaining support.
As an example of the phenomenon such that the above-mentioned hydrogel comprising a polyacrylic acid-type resin markedly inhibits the growth of a plant, there has been reported an experiment wherein distilled water was absorbed into a crosslinked sodium polyacrylate so as to form a hydrogel, and the thus obtained hydrogel was caused to contact seeds of cucumbers and kidney beans for respective periods of time (3, 6, 9, 12, 24 and 48 hours), and then the states of the germination and root origination of the seeds were observed (Kazuo Kawashima, et al., "Influences of Highly Water-Absorbing Polymer Materials on Initial Growth of Crops," Sand Dune Research, 31(1), 1-8, 1984).
As a result of such experiments, it has been reported that the growth of roots was markedly suppressed in the case of cucumber seeds, when they are caused to contact the hydrogel for 36 to 48 hours, and that the inhibition of root growth was also observed similarly in the case of kidney beans. Further, it has been reported that the .alpha.-naphtylamine-oxidizing ability of the root was markedly reduced when the root is caused to contact the hydrogel for 5 hours or more. In this report, such growth inhibition and functional hindrance are presumably attributable to a fact that the plant cannot effectively use the water contained in the hydrogel.
On the other hand, it has been reported that, when rice seeds were sown on a hydrogel which had been prepared by causing crosslinked sodium polyacrylate to absorb water, and then the process of the root origination was observed, serious hindrance in the root origination was recognized (Yorio Sugimura, et al., "Utilization of Highly Water-Absorbing Polymer as Virescence Engineering Material," Techniques of Virescence Engineering, 9(2), 11-15, 1983). In this report, no hindrance in the root origination was observed when the hydrogel was dialyzed with tap water, but the recovery of the root growth was not observed even when the hydrogel was dialyzed with distilled water. In this report, it is presumed that, when the hydrogel is washed or dialyzed with a weak electrolytic solution such as tap water, the water-absorption amount force toward the hydrogel was weakened, and the migration of water from the gel to the root hair is facilitated, thereby to solve the hindrance in the root origination.
It has also been reported an example wherein the elongation of soybean root was markedly inhibited in a soil which had been mixed with a crosslinked sodium polyacrylate hydrogel, as compared with that in the case of a polyvinyl alcohol-type hydrogel (Tomoko Nakanishi, Bioscience & Industry, 52(8), 623-624, 1994). In this reference, this phenomenon is presumably attributable to a fact that the water in the sodium polyacrylate hydrogel is less liable to be utilized for a plant.
As described above, it has heretofore been considered that the inhibition of the growth of a plant in a hydrogel comprising an alkali metal salt of crosslinked polyacrylic acid is attributable to a fact that the water in the hydrogel is not effectively utilized for the plant.
An object of the present invention is to provide a water-retaining support for plant which has solved the above-mentioned problems of the hydrogel water-retaining support encountered in the prior art.
Another object of the present invention is to provide a water-retaining support for plant which has a water-retaining ability comparable to that of the conventional polyacrylic acid-type hydrogel, and does not substantially cause an inhibition in root origination or in root elongation.